Published December 11, 2014
Evaluation of Biological Efficiency of Free-Grazing Beef
Cows Under Semidesert Conditions'
R. M. Kattnig2, J. A. Windel3, J. D. Wallace, and C. C. Bailey
Department of Animal and Range Sciences, New Mexico State University, Las Cruces 88003
ABSTRACT
Effects of cow BW, hip height, and
estimated genetic potentials (EBV) for weaning
weight direct and milk on cow productivity, fecal OM
output, OM intake, and efficiency (kilograms of calf
BWkilogram of OM intake by the cow) were evaluated with 44 free-grazing crossbred cows under
semidesert conditions. Calf BW were measured during
early, mid-, and late lactation. Data were collected in
four periods: Period 1 = late spring (early lactation),
Period 2 = late summer (mid-lactation), Period 3 =
mid-autumn (late lactation), and Period 4 = midwinter (nonlactation). Calf BW increased linearly
with cow BW ( P < .01) in Periods 1, 2, and 3. Fecal
OM output and OM intake increased with cow BW in
Periods 2 ( P< .01) and 4 ( P < .Oil, and on average
( P < .02 j. Overall efficiency decreased with increasing
cow BW ( P< ,041. Taller cows excreted more fecal OM
and had greater OM intake throughout the study ( P <
.02 to P < . 1 1 j . Overall efficiency decreased with
increasing cow hip height ( P < .05). Weaning weight
direct EBV of cows was related linearly to cow BW ( P
< .01 to P < .07) and to calf BW ( P < .01 to P < .07).
Calf weight in all periods increased linearly with milk
EBV ( P < .001). Overall, fecal OM output, OM intake,
and efficiency were not affected by milk EBV.
Key Words: Beef Cattle, Efficiency, Feed Intake, Estimated Breeding Value
J . Anim. Sci. 1993. 71:2601-2607
Introduction
Understanding factors that affect the biological
efficiency of free-grazing beef cattle under specific
environmental conditions is one important step in the
identification of optimal genetic types. Currently, most
range animal evaluation programs are directed a t
total output (BW gain, calf production); little, if any,
attention is given to inputs (forage intake). Ellis et al.
(1982) described a ruminal bolus designed to release
chromic oxide at a constant rate. Measurement of fecal
Cr concentration allows estimation of total fecal
output. Organic matter intake can then be calculated.
This technique provides an opportunity to evaluate
intake and efficiency of utilization of range forages
under free-grazing conditions. Expression of calf
performance relative to intake by the cow should allow
characterization of cattle with the greatest potential
for biological efficiency under specific environmental
conditions.
'Research supported by the New Mexico Agric. Exp. Sta.
2Present address: Dept. of h i m . Sci., Univ. of Arizona, Tucson.
3T0whom correspondence should be addressed: P. 0. Box 30003,
Dept. 3-1.
Received November 2, 1992.
Accepted June 11, 1993.
Our objectives were to evaluate factors that affect
productivity, fecal OM output, OM intake, and biological efficiency of free-grazing cattle under semidesert
range conditions.
Materials and Methods
Study Area. The study was conducted at the New
Mexico State University College Ranch located in the
southern portion of the Jornada Plain, 37 km north of
Las Cruces, NM. Elevation ranges from 1,200 t o 1,350
m. Soil types vary from sandy loam to clay loam. The
study area is typical of a semidesert and is characterized by a bimodal distribution of precipitation with a
summer rainy season from July through September
and variable winter precipitation. Major grass species
include black grama ( Bouteloua eriopoda), burrograss
( Scleropogon breuifolus), threeawn ( Aristida spp.),
and dropseed ( Sporobolus spp.). Forb species vary
widely among seasons but are always a n important
part of the standing crop; typical species present
included winterfat ( Ceratoides lanata), desert holly
( Perezia nana), and leatherweed ( Croton pottsii) .
Principal shrub species were mesquite ( Prosopis
glandulosa) and soaptree yucca ( Yucca elata). Further details about the study area were provided by
Rosiere et al. (1975).
260 1
2602
KATTNIG ET AL.
Available Forage. Standing crop for each of four
experimental periods (early summer, late summer,
fall, and winter) was estimated in a companion study
(Ferrando, 1990). Estimates were as follows: Period
1, standing crop = 551 k g h a (64% grasses, 36%
forbs); Period 2, standing crop = 325 k g h a (74%
grasses, 26% forbs); Period 3, standing crop = 1,077
k g h a (62% grasses, 38% forbs); Period 4, standing
crop = 422 k g h a (82% grasses, 18% forbs). The
proportion of standing crop composed of browse plants
was not included in these estimates; however, Ferrando (1990) estimated that browse plants accounted
for < 100 k g h a .
Experimental Animals. Forty-four crossbred beef
cows with calves a t side were used. Cows were either
3/4 Brangus:1/4 Hereford ( n = lo), 3/4 Hereford:1/4
Brangus ( n = 41, 1/2 Charolais:1/4 Hereford:1/4
Brangus ( n = 5 ) , or 1/2 Simmental:1/4 Hereford:1/4
Brangus ( n = 21). The cows were born in 1981, 1982,
and 1983 and were 6 to 8 yr old at the start of the
experiment. Calves were sired by either Hereford or
Brangus bulls. All cows were reared at the College
Ranch. Cows received only free-choice mineral supplements during the experiment. Individual performance
and pedigree data on cows were summarized using a
single-trait model of the BLUP procedures described
by Quaas and Pollack (1980) to estimate breeding
values ( EBV) for weaning weight direct and weaning
weight maternal for each cow (Winder and Ballard,
1990). The analysis accounted for differences in
expected levels of heterosis by contemporary group
designation. Contemporary groups were designated on
the basis of year of birth, sex of calf, and type of
mating (back-cross or three-breed cross). All progeny
were conceived in multiple-sire pastures. This effect
was accounted for by assuming that each individual
had a unique sire. All connectedness was through
maternal lineage. All cows could be traced back to a
base population. Progeny records reported in this
experiment were not included in the analyses used to
predict breeding values. Mean weaning weight direct
EBV was .4 kg. Values ranged from -9.7 to 8.2 kg.
Mean weaning weight maternal EBV was .4 kg.
Values ranged from -3.1 t o 3.6 kg.
Data Collection. Data were collected on cows and
calves in each of four periods. Period 1 (May 18 to
June 7, 1989) represented the early lactation phase,
Period 2 (July 19 to August 8, 1989) represented the
mid-lactation phase, Period 3 (September 23 to
October 11, 1989) represented the late-lactation
phase, and Period 4 (January 3 to 21, 1990)
represented nonlactation. Mean calf ages were 87,
149, and 215 d a t the beginning of Periods 1, 2 and 3,
respectively. On d 0 of each period, cows were
administered a continuous-release chromic oxide capsule (Barlow et al., 1988). Fecal grab samples were
taken on d 8, 12, 16, and 20 of Periods 1 and 2 and on
d 6, 10, 14, and 18 of Periods 3 and 4. Cow and calf
BW and cow body condition scores ( 1 = emaciated, 9 =
obese; Richards et al., 1986) were taken on each fecal
sampling day in Periods 1, 2, and 3. Hip height
measurements were taken during Period 3 only.
Height was measured as the height of the cow a t the
center of the spine midway between the right and left
ilium. Calves were weaned at the end of Period 3.
Average age a t weaning was 233 d. Cow BW and body
condition scores were taken a t the time of fecal
sampling in Period 4.
Laboratory Analyses. Fecal samples were weighed
and oven-dried (forced-air) at 50°C for 48 h, then
ground in a Wiley mill to pass a 2-mm screen. Fecal
DM and ash were determined using duplicate,
l-g samples by standard procedures (AOAC, 1984). In
preparation for Cr analyses, ash was digested according to procedures outlined by Williams et al. (1962)
using 3 mL of phosphoric acid-manganese sulfate
solution and 4 mL of potassium bromate solution.
Chromium concentration was determined by atomic
absorption spectrophotometry using a nitrous oxideplus-acetylene flame. All samples were analyzed in
duplicate with a maximum acceptable CV of 5%.
Samples with chromium values k 2 SD above or below
the mean value were deleted from statistical analysis
to eliminate chromic oxide capsules that malfunctioned.
Fecal Output, Organic Matter Intake, and Efficiency. Actual daily Cr release of capsules was
determined from total fecal collections (total fecal DM
x fecal Cr concentration) in a companion study using
steers grazing with the test cows (King, 1991). Fecal
OM output was estimated by dividing fecal Cr
concentration (milligrams of Cr/gram of OM) into
release rate of Cr (milligram/day). Organic matter
intake was calculated by dividing estimated fecal OM
output by diet indigestibility. Digestibility estimates
were calculated for each period using in situ OM
disappearance measured in a companion study (King,
1991). Digestibility estimates were 57, 64, 62, and
38% for Periods 1, 2, 3, and 4, respectively. One
digestibility value was used for all cows within each
period. Efficiency was measured in each period as the
mean calf BW for that period divided by the average
estimated daily OM intake of the cow to date. Thus,
Period 1 efficiency reflected OM intake for only that
period; in subsequent periods, efficiency measurements reflected estimated OM intake of the cow for
that period and all previous periods.
Data Analyses. Data were analyzed as a completely
random design using the GLM procedure (SAS,
1985). The experimental model was as follows: Yijk = p
+ Sexi + Breedj + BDk + 61x1 + &X2k + eiju, where Yijk
= response variable (calf BW, cow BW, cow condition
score, fecal output, OM intake, or efficiency measurements); p = mean value for the response variable; Sex
= sex of calf; Breed = breed of sire of calf; BD = Birth
date of calf (number of days since the beginning of the
year); X = average cow BW, cow hip height, cow
weaning weight EBV, or cow milk EBV, and B =
2603
BIOLOGICAL EFFICIENCY OF GRAZING BEEF COWS
Table 1. Means and standard deviations for productivity and efficiency measurements on crossbred cows
Period
2b
la
-
Trait
Cow BW, kg
Calf BW, kg
Cow cond. scoref
Cow hip height, cm
Fecal OM, kgg
OM intake, k$.
Efficiency, kgkg'
3'
-
-
4d
-
Overalle
X
SD
X
SD
X
SD
X
SD
439
92
3.9
7.2
13.8
6.8
45
13
.4
1.6
3.0
1.5
407
140
3.7
42
15
.6
1.1
2.3
2.0
476
213
4.1
125
5.7
12.4
18.6
47
21
.4
3.3
1.7
3.6
2.6
490
4.4
49
.5
.8
1.1
3.0
4.3
8.6
12.6
-
3.5
4.8
21.7
X
454
-
5.2
10.0
-
SD
45
-
-
.7
1.3
-
aMay 18 to June 7, 1989 ( n = 40).
bJuly 19 to August 8, 1989 ( n = 38 for cow BW, calf BW, and cow condition score; n = 37 for fecal OM, OM intake, and efficiency).
'%eptember 23 to October 11, 1989 ( n = 37 for cow BW, calf BW, cow hip height, and cow condition score; n = 31 for fecal OM, OM intake; n
= 29 for efficiency).
dJanuary 3 to January 21, 1990 ( n = 37 for cow BW and cow condition score; n = 36 for fecal OM and OMI; n = 28 for efficiency).
eAverage of four periods (where applicable; n = 34 for cow BW and cow condition score; n = 28 for fecal OM and OM intake).
fl = emaciated, 9 = obese.
gEstimated by dividing fecal Cr (g/kg of OM) into Cr release rate of controlled-release capsule.
hFecal output divided by diet indigestibility.
Calf BW divided by estimated cumulative OM intake.
Table 2. Effect of average cow weight on cow productivity and efficiency measurements
Partial regression coefficients
SE
Traita
n
Linear
SE
OSLb
Period 1 calf BW, kg
Period 2 calf BW, kg
Period 3 calf BW, kg
34
34
34
.10
.14
.18
.03
.04
.06
,008
,003
,006
-
-
-
-
-
-
Period 1 cow cond. score'
Period 2 cow cond. scoreC
Period 3 cow cond. score'
Period 4 cow cond. scoreC
Avg cow cond. scored
34
34
34
34
34
.006
.008
.003
,005
,001
,002
,002
.002
.001
,002
,0001
.08
,008
,0004
-
-
-
Period 1 fecal output,
Period 2 fecal output,
Period 3 fecal output,
Period 4 fecal output,
Avg fecal output, kgd
34
34
29
33
28
.01
.01
.14
.09
.006
.004
,006
.05
.04
.90
.002
34
34
29
33
28
,002
.03
.02
.19
.19
.01
,008
.01
.08
.08
.08
.02
.02
34
34
29
28
.005
-.007
-.31
-.43
.006
.007
.16
.19
.36
.30
.07
.04
Period 1 OMI,
Period 2 OMI,
Period 3 OMI,
Period 4 OMI,
Avg OMI, k$
Period
Period
Period
Period
kge
kge
k$
k$
kgf
kpf
kgf
kgf
1 efficiency,
2 efficiency,
3 efficiency,
4 efficiency,
kgkgg
kgkg
kgk@
kg/k$
,005
.oo
.08
.01
.02
.87
,0008
Quadratic
-
OSLb
-
-
-
-
-
-
-
-
-
-.0002
-.0001
-.0002
-.0002
,0003
,0005
-
,0001
.oooo
-
.0001
,0001
,0002
,0002
.01
.03
.02
.02
.09
.04
apenod 1 = May 18 to June 7, 1989 (early lactation). Period 2 = July 19 to Aug. 8, 1989 (mid-lactation). Period 3 = Sept. 23 to Oct. 11,
1989 (late lactation). Period 4 = Jan. 3 to Jan. 21, 1990 (nonlactation).
bobserved significance level.
'1 = emaciated, 9 = obese.
dAverage of four period measurements.
eEstimated by controlled-release chromic oxide boluses.
fEstimated by dividing fecal OM output by percentage of indigestibility.
gCalf BW divided by OM intake.
KATTNIG ET AL.
2604
regression coefficient. Sex of calf, breed of sire of calf,
and birth date of calf were included in the model to
remove known sources of variation. Both linear and
quadratic effects of average cow BW, cow hip height,
and cow weaning weight EBV and milk EBV were
evaluated initially. If the probability associated with
both linear and quadratic regression coefficients was >
.11, only a linear regression was used in analyses.
Breed of cow was not included in the model. Inclusion
would have limited the scope of these data. Our
objective was to evaluate biological efficiency in a herd
of crossbred cows. Breed differences enhance our
potential to define optimums.
Results and Discussion
Means and SD for all traits measured are listed in
Table 1 by period of measurement.
Cow Body Weight. Partial regression coefficients
and observed significance levels for the regression of
productivity and efficiency measurements on cow BW
are presented in Table 2. Cow BW had a positive
linear effect ( P < .01) on calf BW in Periods 1, 2, and
3. On average, calf weaning weights (Period 3 calf
BW) increased by .18 kgkg of cow BW. Heavier cows
also tended to have higher condition scores in Periods
1, 2, 3, and averaged over all four periods ( P e .O1).
Fecal output estimates displayed a positive linear
relationship with cow BW in Period 2. In Period 4, and
on average, fecal output exhibited linear and quadratic relationships to cow BW ( P < .03 to P < .Ol>,
indicating that heavier cows excreted more OM than
lighter counterparts did. Organic matter intake exhibited a similar relationship to fecal output; intake
generally increased with cow BW. This result was
expected because OM intake was calculated from fecal
OM output. Efficiency in Periods 3 and 4 was related
negatively t o cow BW. These measurements reflect
kilograms of calf BW a t weaning per kilogram of daily
intake by the cow. Efficiency in Period 4 should reflect
year-long eficiency because the denominator reflects
Table 3. Effect of cow hip height on cow productivity and efficiency measurements
Partial regression coefficients
n
Linear
SE
OSLb
kg
kg
kg
kg
34
34
37
37
34
11.2
10.7
11.6
12.0
11.3
.0001
,0001
,0001
.0001
.0001
Period 1 calf BW, kg
Period 2 calf BW, kg
Period 3 calf BW, kg
34
34
37
.9
1.3
1.8
Period 1 cow cond. scoreC
Period 2 cow cond. scoreC
Period 3 cow cond. scoreC
Period 4 cow cond. scoreC
Avg cow cond. scored
34
34
37
37
34
.05
.07
.03
.06
.05
1.5
1.2
1.5
1.6
1.4
.5
.6
.9
.02
.03
.02
.02
.02
Period 1 fecal output,
Period 2 fecal output,
Period 3 fecal output,
Period 4 fecal output,
Avg fecal output, k$
34
34
31
36
28
9.6
.2
.2
6.4
.09
17.9
.4
.3
8.9
.I8
-8.4
4.8
.05
.08
2.2
.03
.06
,0003
9.1
-20
3.0
.06
.06
.0004
.09
,006
,007
4.5
5.6
.12
.15
.07
.ll
.02
.05
Traita
Period 1 cow BW,
Period 2 cow BW,
Period 3 cow BW,
Period 4 cow BW,
Avg cow BW, kgd
Period 1 OMI,
Period 2 OMI,
Period 3 OMI,
Period 4 OMI,
Avg OMI, kgd
Period
Period
Period
Period
kgf
kgf
kgf
kgf
1 efficiencyg
2 efficiencyg
3 efficiencyg
4 effciencyg
k e
kge
kf
kf
34
34
31
36
28
34
34
29
28
-9.4
-.3
-.3
.10
Quadratic
SE
OSLb
.07
.04
.06
.02
.01
.19
.01
.01
.os
,006
.02
aPeriod 1 = May 18 to June 7, 1989 (early lactation). Period 2 = July 19 to Aug. 8, 1989 (mid-lactation). Period 3 = Sept. 23 to Oct. 11,
1989 (late lactation). Period 4 = Jan. 3 to Jan. 21, 1990 (nonlactation).
bobserved significance level.
'3 = emaciated, 9 = obese.
dAverage of four period measurements.
eEstimated by controlled-release chromic oxide boluses.
fEstimated by dividing fecal OM output by percentage of indigestibility.
gCalf BW divided by OM intake.
2605
BIOLOGICAL EFFICIENCY OF GRAZING BEEF COWS
average OM intake throughout the year. Based on
these observations, it seems that under semidesert
conditions, biological efficiency within this time frame
may be maximized with cows of lighter mature BW.
Solis et al. (1988) reported that cows that have the
potential to store fat are more efficient when energy is
limited, whereas cows that have larger protein stores
are more efficient when energy is not limited. Under
our conditions, energy deficiency is common. Lighter
cows would be expected to store energy as fat more
readily than heavier cows because of a reduction in
maintenance requirements. This concept is also supported by the modeling results of Long et al. (1975).
Their deterministic model indicated that smaller cows
excel in both live weight of calf produced and net
income when conditions are somewhat restrictive.
Cow Hip Height. As expected, taller cows tended t o
be heavier cows ( P < .0001; Table 3). Taller cows also
tended to produce heavier calves in Periods 1 ( P <
.07), 2 ( P < .04), and 3 ( P < .06). Taller cows
maintained more condition on average than their
shorter counterparts did; average condition scores
increased by .05 units per centimeter of cow hip height
( P < .01). This observation was unexpected under
these conditions because taller cows would be expected
to have greater BW and maintenance requirements.
Overall, fecal OM output and OM intake tended to
increase linearly ( P < .01 to P < .05) with cow hip
height, except in Period 3. Taller cows seemed to
consume more forage throughout the study, but
biological efficiency seemed t o be negatively related to
hip height. On average, taller cows produced fewer
kilograms of calf weight per kilogram of daily OM
intake. Efficiency decreased by .3 kgkg for each
1-cm increase in cow hip height, indicating that
maintenance of large-framed cows may be detrimental
t o efficient utilization of semidesert rangelands.
Weaning Weight Direct Estimated Breeding Value.
Weaning weight direct EBV are summary statistics
that indicate the genetic potential of the cow for
Table 4. Effect of estimated genetic potential for weaning weight direct (weaning weight EBV)
on cow productivity and efficiency measurements
Partial regression coefficients
n
Linear
SE
40
38
37
37
34
5.4
4.8
4.4
4.6
4.2
1.9
1.9
2.1
2.2
.01
.04
.05
.07
Period 1 calf BW, kg
Period 2 calf BW, kg
Period 3 calf BW, kg
40
38
37
.9
1.4
1.6
.4
.03
.5
.8
,008
.05
Period 1 cow cond. score'
Period 2 cow cond. scoreC
Period 3 cow cond. score'
Period 4 cow cond. score'
Avg cow cond. scored
40
38
37
.03
.28
.85
Traita
Period 1
Period 2
Period 3
Period 4
Avg cow
cow BW,
cow BW,
cow BW,
cow BW,
BW, kg
kg
kg
kg
kg
Period 1 fecal output,
Period 2 fecal output,
Period 3 fecal output,
Period 4 fecal output,
Avg fecal output, kgd
Period 1 OMI,
Period 2 OMI,
Period 3 OMI,
Period 4 OMI,
Avg OMI, kgd
Period
Period
Period
Period
kge
k e
kge
kge
37
34
.04
.02
.03
.oo
.02
.02
.02
-.02
.01
.09
37
31
36
28
.10
.02
.02
.05
kgf
40
k$
kgf
k$
37
31
36
1 efficiencyg
2 efficiencyg
3 eficiencyg
4 efficiencyg
28
40
37
29
28
.17
.03
.04
.20
.13
.14
.21
.10
.18
.05
.07
.05
.06
.09
.14
.16
.66
.26
.96
.99
-.03
-. 10
-.01
.oo
~
OSLb
.48
.06
.78
.56
.15
.02
.ll
SE
.31
.07
.05
.08
.06
Quadratic
,006
2.2
.03
40
OSLb
.74
.66
.13
~
~
~~
~~~
~~
apenod 1 = May 18 to June 7, 1989 (early lactation). Period 2 = July 19 to Aug. 8, 1989 (mld-lactation) Period 3 = Sept. 23 to Oct. 11,
1989 (late lactation). Period 4 = Jan. 3 to Jan. 21, 1990 (nonlactation).
bobserved significance level.
1' = emaciated, 9 = obese.
dAverage of four period measurements
eEstimated by controlled-release chromic oxide boluses.
fEstimated by dividing fecal OM output by percentage of indigestibility.
gCalf BW divided by OM intake.
KATTNIG ET AL.
2606
growth from conception to weaning. On average, cows
with greater weaning weight direct EBV tended to
weigh more throughout the study ( P < .07 to P < .01;
Table 4), and weaning weight direct EBV had a
positive effect on calf BW in all three periods ( P < .05
to P < .01). Calf BW in Period 3 were measured at
weaning. Weaning weights increased linearly a t a rate
of 1.6 k g k g of increase in weaning weight direct EBV
of the cow. Period 2 fecal OM output increased with
weaning weight direct EBV ( P < .06). Organic matter
intake displayed a similar relationship to fecal output
in Period 2. Efficiency did not seem to be affected by
weaning weight direct EBV ( P > .20), which implies
that although cows with more genetic potential for
preweaning growth tend to consume more forage
during certain times of the year, they are able to
maintain their efficiency of conversion to calf BW.
Milk Estimated Breeding Value. Milk EBV estimate
the transmittable genetic potential of the cow for
maternal effects on calf weaning weight. During the
course of the study, cow BW tended to increase with
milk EBV (Table 5 ) . As expected, calf BW increased
with increased genetic potential for milk in the cow ( P
< .0005). Cow condition score was not affected by milk
EBV ( P > .lo). Fecal OM output and OM intake
increased with milk EBV only in Period 2 ( P < .04).
Overall, cows with greater genetic potential did not
seem to consume more forage. Linear ( P < .07) and
quadratic ( P < .04) relationships were observed
between milk EBV and efficiency in Period 1. This
relationship indicates that efficiency increases initially with increased milk production potential, then
reaches a point of diminishing returns. Period 1 was
quite hot and dry with limited forage availability.
Cows were most likely at or near peak lactation.
Richards ( 197 91, using the weigh-suckle-weigh technique, observed that peak lactation occurred a t
approximately 6 to 8 mo postpartum under similar
conditions. These results indicate that under harsh
conditions, maternal milk production levels may be
Table 5. Effect of estimated genetic potential for milk production (milk EBV) on cow
productivity and efficiency measurements
Partial regression coefficients
~~~~~~~~
~
n
Linear
SE
OSLh
kg
kg
kg
kg
40
38
37
37
34
10.9
10.4
9.4
12.2
9.3
5.1
4.9
6.0
6.2
6.2
.04
.04
.13
.06
.14
Period 1 calf BW, kg
Period 2 calf BW, kg
Period 3 calf BW, kg
40
38
37
3.5
4.9
7.4
.9
1.2
1.9
,0003
,0002
.0005
Period 1 cow cond. scoreC
Period 2 cow cond. score'
Period 3 cow cond. scoreC
Period 4 cow cond. score'
Avg cow cond. scored
40
38
37
37
34
.07
.07
.02
.02
.06
.05
.07
.06
.05
.17
.30
.97
.79
.29
Period 1 fecal output,
Period 2 fecal output,
Period 3 fecal output,
Period 4 fecal output,
Avg fecal output, kgd
40
37
31
36
28
-.02
.29
.24
.12
.17
.18
.13
.22
.09
.09
.89
.04
.27
.19
.07
40
37
31
36
28
.02
.64
.58
.15
.34
.34
.27
.47
.13
.17
.96
.03
.22
.25
.06
40
37
29
28
.29
.12
.09
.10
.16
.23
.35
.41
.07
.62
Traita
Period 1 cow BW,
Period 2 cow BW,
Period 3 cow BW,
Period 4 cow BW,
Avg cow BW, kgd
Period 1 OMI,
Period 2 OMI,
Period 3 OMI,
Period 4 OMI,
Avg OMI, kgd
Period
Period
Period
Period
kgf
kgf
kgf
kgf
1 effciencyg
2 eficiencyg
3 eficiencyg
4 efficiencyg
kge
kge
kge
kge
.05
Quadratic
SE
OSLh
.80
.a2
aPeriod 1 = May 18 to June 7. 1989 (early lactation). Period 2 = July 19 to Aug. 8, 1989 (mid-lactation).Period 3 = Sept. 23 t o Oct. 11,
1989 ( l a t e lactation). Period 4 = Jan. 3 to Jan. 21, 1990 inonlactation).
hobserved significance level.
'1 = emaciated, 9 = obese.
dAverage of four peiiod measurements.
eEstimated by controlled-release chromic oxide boluses.
fEstimated by dividing fecal OM output by percentage of indigestibility.
gCalf BW divided by OM intake.
BIOLOGICAL EFFICIENCY OF GRAZING BEEF COWS
identified that would result in maximized efficiency.
No relationships between efficiency and milk EBV
were observed in other periods.
Increased milk production potential has been associated with elevated maintenance requirements
even when the cow is in a state of nonlactation
(Ferrell and Jenkins, 1984). Under restrictive conditions, efficiency may be expected to decline if nutrient
intake is inadequate to meet lactation and maintenance requirements. We were not able to demonstrate any consistent relationship between milk
production potential (measured as milk EBV) and
efficiency. This may be due to error in estimation of
breeding values or to lack of sufficient variation in
milk production potential among the cows.
These data indicate that under semidesert conditions, optimal biological efficiency may be achieved by
using cows that are small in both weight and stature.
Genetic potential for preweaning growth did not seem
to affect efficiency within the range of these data. As
environmental conditions worsen, increased milk
production may eventually result in decreased biological efficiency. The number of observations in this
study was limited. More data are needed to pinpoint
optimal size and level of genetic potential for semidesert rangelands.
Implications
Tailoring cow type to restrictive environments
might improve eficiency of conversion of forage to calf
weaning weight. Our data, however, only reflect
estimated biological efficiency during a single year,
and as such they reflect neither lifetime efficiency nor
economic efficiency. Optimal economic returns may
occur at something less than maximal biological
efficiency. For instance, maximal biological efficiency
may occur with cow weights that are too low t o allow
production of calves that reach acceptable slaughter
weights. More research is needed to establish not only
optimal size and genetic potentials for maximal
biological efficiency, but also maximal economic efficiency.
2607
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